1) Field of the Invention
The present invention relates to a process for producing .epsilon.-caprolactone. .epsilon.-caprolactone is useful as a raw material of .epsilon.-caprolactam which is used as a raw material of nylon resin, or a raw material of polyurethane.
2) Prior Art
.epsilon.-caprolactone is a high boiling point liquid with excellent reactivity and used as a raw material of polyesterpolyol, urethane resin, etc.
Thus, polyesterpolyol for polyurethane and high molecular weight polyesterpolyol which are produced from .epsilon.-caprolactone, are used as blend material for improvement of properties of various resins.
When polyesterpolyol obtained from .epsilon.-caprolactone is blended with polyethylene, gloss transparency and blocking preventive ability are improved. When it is blended with polyvinyl chloride, low temperature embrittlement of polyvinyl chloride is improved. However, properties of .epsilon.-caprolactone as raw material, particularly, appearance (coloration, etc.) of .epsilon.-caprolactone often have an influence also upon appearance, weather resistance, etc., of blended various resins.
As treatments for such .epsilon.-caprolactone, particularly, heretofore, improvement of purity by repeat of distillation, preservation under nitrogen atmosphere and addition of a stabilizer such as p-methoxyphenyol, tridecylphosphite, 2,6-ditertiary-butyl-4-methylphenol, etc., have been conducted. Japanese Patent Kokai (Laid-open) No. 5-1054 describes to use a hydrotalcite compound as the adsorbent.
.epsilon.-caprolactone is produced by oxidation of cylohexanone. As processes for producing .epsilon.-caprolactone by oxidation of cyclohexanone, there are a co-oxidation process wherein both cyclohexanone and aldehyde are oxidized and a peracid oxidation process using organic peracid as an oxidizing agent.
When conventional distillation processes are applied for separation of the intended substance from a reaction mixture obtained by such oxidation of cyclohexanone, polymerization loss of .epsilon.-caprolactone becomes large and the yield is lowered.
The loss of .epsilon.-caprolactone in the distillation step is due to high boiling substances contained in the reaction product. Japanese Patent Kokai (Laid open) No. 53-34789 describes a process wherein, in order to prevent the loss, the high boiling substances are removed with a thin film type evaporator provided prior to the distillation step of unreacted cyclohexanone. However, in the above-mentioned process an expensive evaporator is necessary in addition to heat evaporator attached to a distillation column and both cost of apparatus and energy is increased.
Japanese Patent Kokai (laid-open) No. 57-42684 describes to distill off low boiling substances from a reaction mixture obtained by organic peracid oxidation of cyclohexanone and then to distill in an apparatus provided with a single pass type thin film evaporator. However, in the above-mentioned process, a peracid oxidation process using expensive organic peracid is applied. When the co-oxidation process is applied to the above-mentioned process instead of the peracid oxidation process, loss of .epsilon.-caprolactone is large since polymerization remarkably occurs. Further in the above-mentioned process a severe burden in the aspect of economy is imposed since it is necessary to apply the expensive single pass type thin film evaporator as a reboiler.
As treatment of .epsilon.-caprolactone, as described above, when distillation is repeated, its appearance becomes colorless and transparent just after the distillation, but coloration often occurs during long time preservation. Also in preservation under nitrogen atmosphere, sufficient improvement for coloration cannot be expected. Addition of a stabilizer causes deterioration of appearance of urethane resin which is produced using lactonepolyol. Use of an adsorbent causes a problem that it is considerably expensive.
Although various improvements of process for loss of .epsilon.-caprolactone in the distillation step have been tried, a severe burden in the aspect of economy is imposed since expensive single pass type thin film evaporator is applied. Therefore, further improvement is required.